Machines that have relatively movable parts include bearings. In those situations where there is direct contact between relatively moving parts, consideration must be given to the problem of wear. Typically, bearing materials are selected and/or a lubricant is used so that adequate service life is obtained and so that the risk of catastrophic failure is minimized. In some machines, lubrication is provided by process fluid. One example of such a machine is a sealless pump (also referred to as a canned motor pump or magnetic drive pump) which is used to pump fluids that must be isolated from the ambient environment. In one type of sealless pump, a motor stage and a pump stage are disposed within a single housing and include a common rotor coupled to a pump impeller. The rotor is supported by radial journal and axial thrust bearings lubricated with process fluid. These bearings are made of a relatively soft carbon or wear resistant ceramic material. Typically, lubrication by the process fluid is marginal, and intermittent dry operation is encountered. While short periods of operation without substantial lubrication can be tolerated, longer periods of dry bearing operation can result in a costly and potentially hazardous situation.
Conventionally, a wattmeter is used to measure the power expended by the motor. If process fluid is absent in the pump stage, then the power consumed by the motor drops, and this condition is sensed by the wattmeter to detect the probability of a dry bearing condition. However, a situation can arise wherein process fluid is present at the pump impeller but is not present at one of the bearings. In this case, the wattmeter would fail to indicate the dry bearing condition.